Tool Briefing: Charge Light Check-out

Some tools and tricks for charging system analysis

The trouble code in this customer’s vehicle, P1638, indicates incorrect charging voltage. Any time you examine a charging system that’s controlled by the Powertrain Control Module (PCM), the very first tool you need is a service information system. Different battery technologies require different charging voltage, and the charging system may be programmed to work only with the factory-installed battery type. For conventional flooded lead/acid batteries, the charging system would be programmed to produce a nominal 14.5 volts. With other batteries, such as Absorbed Glass Mat (AGM), Valve Regulated Lead/Acid (VRLA), Gel Cells and other types of sealed batteries, different charging voltages may be required. Before doing anything, check your vehicle service information system for battery specifications and charging system specifications.

The next step is to check the condition of the battery. Checking voltage is a good place to start, but a battery load test is just as important. The carbon pile load tester, which has been used for over a century, is a circuit that converts electrical energy into heat. We can vary the resistance in the circuit to vary the current draw or ‘load’ placed on the battery. Newer testers can also test an alternator’s voltage output, current output and the diodes.

Load testers use an inductive pick-up to read battery and alternator current: when a coil of wire is wrapped around a straight wire, current passing through the straight wire will induce a voltage in the coil. That voltage ‘picked up’ is directly proportional to the current in the straight wire, so by measuring the induced voltage, the tester can calculate the current through that circuit.


With a little practice and imagination, a standard inductive pick-up current probe can be used for testing low current circuits too. Wrap a jumper wire ten times around your hand to make ten loops of wire. Secure the loops together, add the jumper wire to the circuit and clamp the current probe through the loop. The probe will now see ten times the current running through the circuit.

VAT Testing

The easiest way to test a charging system is under load with a Volt/Amp Tester (VAT). There are many on the market, and several are small enough to carry in the vehicle while it’s being driven. Some will generate a printout of diagnostic results, and the same data can also be downloaded to a computer.

On some of the newest vehicles, the PCM will command almost no charge at idle or when there is almost no electrical load. A VAT tester connects to the battery terminals and draws current, forcing the alternator to increase current output to keep the battery charged. Not all VAT testers are capable of putting a full load on the alternator, but when combined with an oscilloscope ripple test, even 50 percent load is enough to tell the story.

Duty Cycle Test

If the alternator is capable of producing current, measuring the field duty cycle will tell you if the alternator is being commanded to produce current. That command comes from the PCM, which controls the ‘on time’ of the alternator field current, and you can see it with a Digital Volt/Ohm Meter (DVOM) that reads duty cycle. When field current is turned on continuously, that’s a 100 percent duty cycle and the alternator will produce its maximum rated current output. The alternator will produce half its rated output when field current is turned on for only 50 percent of one full cycle.


What’s a ‘full cycle?’ When a DVOM is first connected and set to read duty cycle (Hertz or Hz%), it reads the minimum and maximum voltage on that circuit. It sets a trigger point half way between the min and max readings. One full excursion above that trigger point, then below it, and back up to it again, is one full cycle. The percentage of each cycle that the voltage is above that mid point is defined as ‘on time.’ On some meters, you can manually set the trigger point.

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